1. Field of the Invention
The present invention relates to electrophotographic image forming apparatuses, and more particularly, to techniques of reducing variations in density and tint of an image.
2. Description of the Related Art
Generally, in electrophotographic image forming apparatuses, optimum image formation conditions depend on changes over time in components such as the photosensitive member, the development unit, etc., and environmental conditions (temperature and humidity) during image formation. Therefore, in such image forming apparatuses, variations in density and tint of a formed image are, for example, reduced by the following known technique: a test pattern image (patch image) of each color is formed on the photosensitive drum or the intermediate transfer member, and based on the result of measurement of the density, the image formation conditions are controlled to achieve the reduction. This technique enables the image forming apparatus to maintain a stable or consistent quality of image formation.
However, it takes time to perform such a control (image formation stabilizing control). For example, if the control is performed every time image formation has been performed on a predetermined number of sheets, a print job being executed may be interrupted. For example, if the image formation stabilizing control is performed in the middle of continuous image formation (printing) of a large quantity of recording materials, the period of time during which the image formation stabilizing control is performed is a dead time to the user. On the other hand, if the frequency of the image formation stabilizing control is reduced, the quality of image formation deteriorates.
To address such a problem, Japanese Patent Laid-Open No. 2007-219089 proposes a technique of stabilizing an image density by performing, during image formation, an image formation stabilizing control based on an estimation process without forming a test patch. Japanese Patent Laid-Open No. 2010-102317 proposes a feedforward control technique of stabilizing an image density by estimating the charge amount of toner particles based on an estimation model and controlling image formation conditions, such as a contrast potential or gradation conversion conditions during image formation, to suppress the fluctuation in the density of an output image in real time.
However, when the control such as in Japanese Patent Laid-Open No. 2007-219089 supra or Japanese Patent Laid-Open No. 2010-102317 supra is performed, ideal density characteristics can be achieved in a target density region, but an error may occur in a control (correction) of the density characteristics in the other density regions. For example, in Japanese Patent Laid-Open No. 2007-219089 supra, development contrast is corrected based on changes in temperature and humidity in the image forming apparatus or changes in the charge amount of toner in the development apparatus, but even if the density characteristics are corrected in a portion of all density regions, such as a high density region etc., the other density regions are not necessarily able to be corrected.
In Japanese Patent Laid-Open No. 2010-102317 supra, the toner concentration or the toner charge amount is estimated, and based on the result of the estimation, a look-up table (LUT) corresponding to a gradation correction table is corrected to correct the density characteristics of all density regions. However, a change in an image which is determined by the toner concentration or the toner charge amount highly contributes to a high density region, and therefore, it is difficult to accurately correct the density characteristics of a low density region based on the result of the estimation.